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The Sun's Meridian Transit

latitude    longitude

Enter latitude in decimal degrees,
and press return key,

enter longitude in decimal degrees,
and press return key.

gnomon height

enter gnomon height (meters),
and press return key.

Use the keys "y", "m", or "d" to increase the year, month, or day,
or shift key and
"y", "m", or "d" to decrease the year, month, or day.

Use the command key and "u" or "d" to move up or down.
Use "Reset" from the Details menu to reset the position and the zoom.
aperture hole

enter aperture diameter (in mm),
and press return key.

year century
is the year 2009

Select "Write Data" or "Write Table" from the Details menu to open data windows.

The meridian in Basilica di Santa Maria degli Angeli e dei Martiri in Rome

The meridian in the Cathedral of Milan

The meridian of St. Sulpice in Paris

Meridian line sundials were the earliest means of checking the accuracy of calendars. A special form was developed in Italy during the fifteenth century. Instead of using the shadow of an obelisk, the sun was allowed to shine through the wall or roof of a building onto a meridian line on the floor inside, and the point of light was used as the indicator.

solstice equinox

disk of

The diameter S of the disk of light is mainly determined by the apparent diameter α of the Sun:
S = D·tan α

The diameter a of the aperture and an amount due to diffraction (2·D·1.22·λ/a) has to be added to S.

Meridian lines in churches
(adapted from M. Catamo, C. Lucarini):

Year of
City Church/Building Height of
gnomon hole
1467 Florence S. Maria del Fiore
90.11 m
P. Toscanelli
1636 Marseille College of Oratory 17 m
P. Gassendi
1655 Bologna San Petronio
27.07 m
G. D. Cassini
1702 Rome S. Maria degli Angeli
20.34 m
F. Bianchini
1743 Paris S. Sulpice
26 m
Le Monnier
1786 Milan Cathedral 23.82 m
A. De Cesaris, G. Reggio
1791 Naples National Archaeol. Museum 14 m
G. Cascella
1841 Catania Monastery S. Nicolol'Arena
23.92 m
C. F. Peters, W. Sartorius
1794 Palermo Cathedral C11.78 m
G. Piazzi
1895 Modica S. Giorgio
C14.18 m
A. Perini

Web Links

Santa Maria degli Angeli e dei Martiri (Wikipedia)

A Guide to “The Sun in the Church” by J. L. Heilbron

L'Almanacco Astronomico - La vostra eclisse

Der Meridian im Dom von Mailand

Churches as scientific instruments (J. L. Heilbron)

J. L. Heilbron: The Sun in the Cathedral, Cathedrals as Solar Observatories,
Harvard University Press, 1999.

Marco Catamo, Cesare Lucarini: Il Cielo in Basilica. La Meridiana delle Basilica di Santa Maria degli Angeli e dei Martiri in Roma,
A.R.P.A. Edizioni AGAMI, Roma 2002.

Updated: 2023, Oct 06